Treated biosolids from the Nelson Regional Sewage Treatment Plant have been applied to a 1000-ha Pinus radiata forest plantation at Rabbit Island near Nelson City since 1996.

The objectives of the trial are to monitor the ecological and environmental impact of repeated application of biosolids on the pine plantation ecosystem, and to determine sustainable application rates with minimum impact on the receiving environment.

This trial is unique both nationally and internationally due to the comprehensive and long-term assessment.

Since the establishment of this research trial, tree nutrition, growth and wood properties have been assessed along with a number of environmental variables, such as soil and groundwater quality over the 18-year period.

The research findings from this long-term forest field trial have supported and informed management practices for sustainable land application of biosolids, and provided direct evidence for regional councils to make informed decisions during the resource consent application process.

Beneficial use of biosolids as a supplemental fertiliser and soil amendment is one of the most common options for biosolids management. In New Zealand, application of biosoilds on forest land is preferred than on agricultural land because it can reduce the risk of contaminants entering the human food chain and it can also increase tree growth and subsequent economic returns. Treated biosolids from the Nelson Regional Sewage Treatment Plant have been applied to a 1000-ha radiata pine forest plantation at Rabbit Island near Nelson City since 1996. A research trial was established on the site in 1997 to investigate the long-term effects of biosolids application on soil and groundwater quality, tree nutrition and growth. Biosolids have been applied to the trial site every three years (1997, 2000, 2003, 2006, 2009 and 2012, respectively) at three application rates: 0 (Control), 300 (Standard loading rate) and 600 (High loading rate) kg N per hectare. Tree nutrition status and growth are monitored annually, groundwater quality quarterly and soil properties every three years to determine both the risks and benefits and sustainable application rates as well. The latest radiata pine tree growth measurement and wood core sampling at the Rabbit Island biosolids research trial were completed in 2015 (Fig. 1). The effects of N input from biosolids application on forest carbon (C) sequestration were quantified using the C-Change model from annual plot measurements of stand density, stem height and diameter, and annual breast height wood densities obtained from increment cores.

At age 24 years (2015), the P. radiata trees treated with the Standard biosolids loading rate had sequestered significantly more carbon than untreated trees but there was no additional increase in sequestration for trees receiving the High loading rate (Table 1). The total carbon sequestered in fertilised trees averaged 260±10 t C ha-1 (mean of Standard and High loadings with 95% confidence interval) compared with 220±13 t C ha-1 in unfertilised trees, a difference of 40±16 t C ha-1 or 18±7%. Stem volume of fertilised trees was 23±7% higher than unfertilised trees but stem wood density of fertilised trees was 11±8 kg m-3 or 2.5±1.9% lower than unfertilised trees. Thus, in terms of carbon sequestration, the positive effects of N addition from biosolids application on stem volume growth greatly outweighed the negative effects of reduced wood density. As is typical in young P. radiata stands, the great majority of carbon is sequestered in the above and below ground biomass pools with only a small component in dead wood and litter pools (Table 1). Annual treatment means for carbon sequestration are shown in Fig. 2.

Table 1. Carbon stored in the P. radiata trees, stem volume and stem wood density at age 24 years by treatment. Carbon is shown for AGB (above ground biomass), BGB (below ground biomass) DWL (dead woody litter), and FL (fine litter). Values in a column followed by the same letter do not differ significantly (LSD test, α=0.05).

Biosolids loading

Carbon (t C ha-1)

Stem volume (m3 ha-1)

Stem wood density (kg m-3)

Total

AGB

BGB

DWL

FL

Control

220 a

175 a

36 a

1 a

8 a

595 a

418 a

Standard

259 b

206 b

43 b

2 a

10 b

716 b

413 a

High

261 b

207 b

43 b

1 a

9 b

744 b

403 b

Fig. 2. Annual means by biosolids treatment showing total carbon sequestration. The following symbols and lines are used for each biosolids loading rates: Control - dots and solid line; Standard rate – stars and dashed line; High rate – triangles and dotted line. Error bars show standard errors.

In summary, trees applied with biosolids had sequestered 40 t C ha-1 more than unfertilised trees, an increase of 18%. Biosolids application increased stem volume by 23% but reduced stem wood density by 2.5%. Most of the increased C sequestration occurred between age 6 when the trial commenced and age 17 years and the Standard rate gave the same increase in C sequestration as the High rate. This suggests that 2-3 applications of the Standard rate would have been sufficient to achieve the increased C sequestration, implying an applied N to C conversion ratio of 43-65 kg C kg-1 N, similar to ratios estimated from forest fertiliser trials in the Northern Hemisphere. This suggests that N input from biosolids application accelerates the rate of increase of needle mass in the developing forest canopy of P. radiata, but does not necessarily affect the final level of foliage mass in the closed canopy. It is likely that N fertilisation will become more widespread under the New Zealand Emissions Trading Scheme which encourages forest management practices that improve C sequestration in young forests.

Science component of future research

The important future research questions that need to be addressed and can only be tested in this unique long-term biosolids research trial include:

1. What will be the influence of long-term biosolids application on soil microbial community structure and enzymatic functions? How will these changes affect the carbon (C) and nitrogen (N) transformations and storages in the soil and tree biomass, and their impact on forest soil fertility and productivity?

2. Will the repeated application of biosolids enhance the stability of soil organic matter in sandy soil and increase the long-term C sequestration in the soil for beneficial reuse of biosolids in forest land?

3. How can we predict the long-term fate of biosolids-derived heavy metals and organic contaminants in the receiving environment and, their ecological and environmental consequences?

We plan to use an integrated approach to investigate how long-term application of biosolids to unproductive land can enhance soil fertility and functions, pine forest productivity and ecosystem functioning. The impact of biosolids application on environmental quality (soil and groundwater) will also be assessed based on the long-term forest trial on unproductive land at Rabbit Island.

Results from this long-term trial will help waste managers/land owners in Nelson in particular and other regions in general to make informed decisions on sustainable biosolids application and resource consent application, and will be applied to the case-study sites nationally due to the science excellence.

This long-term research trial is made possible because of the excellent collaboration and financial support of the Nelson Regional Sewerage Business Unit (NRSBU)/NCC, TDC, PF Olsen, Scion and CIBR.

Contact Jianming Xue for further information on the Rabbit Island trial.